[LAD] Software sound

Nick nickycopeland at hotmail.com
Mon Sep 1 10:53:52 UTC 2014

> Date: Mon, 1 Sep 2014 09:35:25 +0100
> From: w.boeke at upcmail.nl
> To: linux-audio-dev at lists.linuxaudio.org
> Subject: Re: [LAD] Software sound
> an interesting question might be: do you 
> hear a difference between a Moog- and a Chamberlain filter? And if the answer is yes, then isn't there a simpler way to reach the desired result?

Yes, you definitely hear the difference between these two algorithms. The best way I could describe the difference would be 'warmth', 'character.'

You will also see very different performance profiles for the two algorithms, the Chamberlains are a lot more efficient.

Life is full of choices but they are all trade-offs. I might stick my neck out and suggest that it could be possible to optimise Fons' design to factor out a few divisions. I did the same (with help and advice from other people) on a Huovillainen filter and there are a lot of commonalities between the two algorithms. The result on the Huovilainen algorithm was the same sound quality, smaller CPU footprint but still not much closer to the efficiency of the Chamberlain, almost to the point that it was not worth the effort. Other more efficient optimisations are possible using approximation for the tanh() function but they do affect the filter characteristics.

As a side note to this, the Moog design was patented as a four stage ladder filter using transistors. Diverse manufacturers copied the design, the most notable that I know of was ARP with their Odyssey synth. Moog sued them for abuse of patent, ARP backed down and agreed to pay royalties for use of the design (in all subsequent sales of their synth).
The point I wanted to make is some people mentioned that the design used diodes rather than transistors. Well the Moog design did not use diodes but there were several other manufacturers who took the Moog design and manipulated it to run with diodes hence circumventing the patent - if you have seen such designs then it is possible they were released due to not being in violation. Pretty sure the muSoniq Sonic-5 did this and here is another rub: muSoniq was started by an ex-Moog employee (Zumchek) and muSoniq also later bought the rights to the Moog company after Bob pretty much went bankrupt.

Regards, nick
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